Overloading the Comma Operator

 In C++, we can overload the comma operator using Operator Overloading. For Example: For "Send the query X to the server Y and put the result in variable Z", the "and" plays the role of the comma. The comma operator (, ) is used to isolate two or more expressions that are included where only one expression is expected. When the set of expressions has to be solved for operands, only the rightmost expression is considered.

Examples:  

Input: x = (y = 5, y + 2)
Output: x = 7, y = 5  
Explanation:
In the above expression:
It would first assign the value 5 to y, and then assign y + 2 to variable x.
So, at the end 'x' would contain the value 7 while variable 'y' would contain value 7.

Operators Resistant to Overloading are as follows:

• Scope resolution operator(::)
• sizeof()
• member selector(.)
• member pointer selector(*)
• ternary operator(?:)

Syntax:

return_type class_name::operator op(argument_list)
{
    // body     
}
where,
1) return_type: is the type of value returned by the function.
2) class_name: is the name of the class.
3) op: is an operator function where op is the operator being overloaded, and the operator is the keyword.  

Rules for Operator Overloading:

• Existing operators can only be overloaded, but the new operators cannot be overloaded.
• The overloaded operator contains at least one operand of the user-defined data type.
• The friend function can't be used to overload certain operators. However, the member function can be used to overload those operators.
• When unary operators are overloaded through a member function take no explicit arguments, but, if they are overloaded by a friend function, takes one argument.
• When binary operators are overloaded through a member function takes one explicit argument, and if they are overloaded through a friend function takes two explicit arguments.

In the below code, although, each expression is evaluated by the compiler, the values of left-hand expression are discarded. Finally, The value of the right-hand operation is returned by the function. This triggers the overloaded comma operator to function similarly to its default operation.

// C++ program to illustrate the
// overloading the comma operator
#include <iostream>
using namespace std;

// Comma class
class comma {
    int x, y;

public:
    // Default Constructor
    comma() {}

    // Parameterized Constructor
    comma(int X, int Y)
    {
        x = X;
        y = Y;
    }

    // Function to display the value
    void display()
    {
        cout << "x=" << x << " ";
        cout << "y=" << y << " ";
    }

    comma operator+(comma ob2);
    comma operator, (comma ob2);
};

// Function to overload the + operator
comma comma::operator+(comma ob2)
{
    comma temp;

    // Update the value temp
    temp.x = ob2.x + x;
    temp.y = ob2.y + y;

    // Return the value temp
    return temp;
}

// Function to overload the, operator
comma comma::operator, (comma ob2)
{
    comma temp;

    // Update the value temp
    temp.x = ob2.x;
    temp.y = ob2.y;

    // Print the value
    cout << "x=" << ob2.x << " "
         << "y=" << ob2.y << endl;

    // Return the value temp
    return temp;
}

// Driver code
int main()
{
    // Initialize objects
    comma ob1(10, 20), ob2(5, 30), ob3(1, 1);

    ob1.display();
    ob2.display();
    ob3.display();

    cout << endl;

    ob1 = (ob2 + ob2, ob1, ob3);

    // Displays the value of
    // ob3 (Rightmost expression)
    ob1.display();

    return 0;
}

x=10 y=20 x=5 y=30 x=1 y=1 
x=10 y=20
x=1 y=1
x=1 y=1

Below is another example where the comma (, ) operator is overloaded in the class named Coords3D. 

• The class has 3 internal hidden variables x, y, z.
• Get() method which is the access method to get the values of x, y, z.
• The operator function operator, (), which overloads the operator ', '.

Below is the program for the same:

// C++ program to illustrate the
// overloading for comma operator

#include <iostream>
using namespace std;

// The class that defines the
// coordinates of a point in space
class Coords3D {

private:
    double x, y, z;

public:
    // Default Constructor
    Coords3D() { x = y = z = 0; }

    // Parameterized Constructor
    Coords3D(double x, double y,
             double z)
    {
        this->x = x;
        this->y = y;
        this->z = z;
    }

    // Function for updating the value
    // ofx, y, and z
    void Get(double& x, double& y,
             double& z)
    {
        x = this->x;
        y = this->y;
        z = this->z;
    }

    // Function to overloaded the
    // operator, (comma)
    Coords3D operator, (Coords3D obj)
    {
        Coords3D tmp;

        // Update the value of temp
        tmp.x = obj.x;
        tmp.y = obj.y;
        tmp.z = obj.z;

        // Return the value of temp
        return tmp;
    }
};

// Driver Code
int main()
{
    double x, y, z;

    // Instances of class Coords3D
    Coords3D c1(1, 3, 5);
    Coords3D c2(2, 4, 6);
    Coords3D c3;

    // Invoke the operator function
    // c3.operator, (c2) into c3
    // is saved c2
    c3 = (c1, c2);

    // Get the value of x, y, z
    c3.Get(x, y, z);

    // Print x, y, and z
    cout << "x = " << x << endl;
    cout << "y = " << y << endl;
    cout << "z = " << z << endl;

    // Create another instance
    Coords3D c4(10, 15, 20);

    // c3 <= c4
    c3 = (c2, c1, c4);

    // Checking
    // x = 10, y = 15, z = 20
    c3.Get(x, y, z);

    cout << endl;

    // Print x, y, and z
    cout << "x = " << x << endl;
    cout << "y = " << y << endl;
    cout << "z = " << z << endl;

    return 0;
}

x = 2
y = 4
z = 6

x = 10
y = 15
z = 20